Modern communication systems, such as e.g. UMTS (Universal Mobile Telecommunications System), LTE (Long Term Evolution) and LTE-A (Advanced) cause a significantly higher amount of out-of-band interference into adjacent frequency bands than earlier “legacy” communication systems, such as e.g. GSM (Global System for Mobile communications). Such out-of-band interference is also sometimes referred to as “interference leakage”. The main reason for this out-of-band interference is that modern systems typically employ larger, or wider, bandwidth(s) than legacy systems, and that it is more difficult to develop filters which can cut the out-of-band emissions for a relatively large bandwidth than for a relatively narrow bandwidth.
This significantly higher amount of out-of-band interference or emissions may result in violation of e.g. regulations concerning interference caused to other systems, which employ frequency bands adjacent to the frequency band(s) employed by the interference generating, or “disturbing” systems. Examples of systems using spectrum bands which are adjacent to, or are anticipated to be adjacent to, the frequency bands used by e.g. UMTS, LTE and LTE-A in some countries, and thus may be subjected to out-of-band interference from these systems, are e.g.:                the radio navigation and communication systems between airplanes and the ground, such as DME (Distance Measuring Equipment) (962-1213 MHz) and future L-DACS (L-band Digital Aeronautical Communication System) (960-1164 MHz)        the communication/control system used for communication with and control of trains GSM-R (Railway) (873-880/918-925 MHz in Europe)        
For example, in Europe, an LTE system may use the 900, 1800, and/or 2600 MHz bands. Thus, e.g. an LTE system DL (DownLink) transmission in the 900 MHz frequency band may cause interference to transmissions within the DME or L-DACS systems used for communication with, or control of, e.g. airplanes. Such a scenario is illustrated in FIG. 1. Thus, in a worst case scenario, the out-of-band interference from the LTE DL transmission may e.g. interrupt or disturb important airplane control commands.
A number of solutions have been proposed to mitigate interference between systems using adjacent frequency bands. Most of these proposed solutions focus on the use of a (static) low transmit power in the system causing out-of band interference, or on the use of fixed so-called “guard bands” between the adjacent frequency bands used by different systems or operators. By guard band is meant a frequency band which is not used for communication, but as a buffer for out-of-band interference caused by the systems employing the frequency bands surrounding the guard band. The wider frequency bands used by the systems, the wider guard bands are needed to avoid interference between the systems.
These proposed prior art solutions, however, may cause e.g. coverage problems when the (static) transmit power is too low, and, may further be inefficient e.g. in terms of radio resource usage, by the use of unnecessarily large guard bands based on a worst-case scenario. Further, bandwidth is a scarce resource that is very valuable to the respective authorities or organizations controlling the frequency spectrum in each country. Thus, at least for economical reasons, reserving wide guard bands between systems using adjacent frequency bands is not an attractive idea.